NIRCam Filters for Coronagraphy
The sizes of the three round occulting masks are optimized for coronagraphic observations at λ ~ 2.1, 3.35, and 4.3 µm. At these wavelengths, the inner working angle IWA ~ 6 λ/D, where D is 6.5 m, the effective diameter of the JWST entrance aperture. IWA is the minimum-detectable apparent separation between the (occulted) bright source and the nearby faint source of scientific interest, assuming photometric criteria for detection are satisfied.
The two bar-shaped coronagraphic masks have tapered widths to accommodate ranges of short or long wavelengths with IWA ~ 4 λ/D.
When using each mask, a user may choose from multiple filters. For the round masks, the chosen filter may deviate from the optimal wavelength with some restrictions.
The NIRCam coronagraphs were not designed for optimal performance below 2 µm. Chromatic aberrations are induced in NIRCam's short wavelength channel by the optical wedges included in the Lyot stop elements used to project the occulting masks onto the detectors. These aberrations increase the PSF size along one axis, especially for wide filters, complicating PSF subtraction of the reference star. Furthermore, the coronagraph substrate antireflective coating has poor transmission below 1.9 µm (Figure 1).
Ground-based coronagraphs typically offer better performance (lower chromatic aberrations, wavefront errors, and IWAs) below 2 µm, except near 1.8 µm where the atmosphere is opaque. Therefore, NIRCam coronagraphy is not available below 1.7 µm.
The transmission of the occulting mask changes as a function of radius and differs for each occulting mask.
Table 1. Permitted filters for use with each coronagraphic occulting mask
|Description||Small round||Medium round||Large round||Narrow bar||Wide bar|
|Nominal wavelength(s)||2.10 µm||3.35 µm||4.30 µm||2.1 µm (center)|
|4.6 µm (center)|
0.40″ (2.1 µm)
0.57″ (3.35 µm)
0.87″ (4.30 µm)
0.23″ (1.82 µm)
0.25″ (2.00 µm)
0.27″ (2.12 µm)
|F250M F277W F300M F335M F356W F360M F410M F430M F444W F460M F480M|
For filters in orange, the transmission of the coronagraph optical mount (COM) can have a substantial impact on the effective wavelength of the observations. For example, the COM transmission increases from 48% at 1.8 µm to 88% at 1.9 µm.
Throughputs for all filters are available at NIRCam Filters.
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Krist, J,. et al. 2010, SPIE, 7313J
The JWST/NIRCam coronagraph flight occulters